1. Field of the Invention
This invention relates generally to an apparatus and method for treating and assessing the efficacy of such treatment of CHF patients, and more particularly to an improved cardiac rhythm management device incorporating circuitry for sensing respiratory, heart rate and/or activity-related parameters.
2. Discussion of the Related Art
xe2x80x9cCardiac Pacemakersxe2x80x9d and xe2x80x9cCardiac Rhythm Managementxe2x80x9d (CRM) devices are both used as generic terms for pacemakers and defibrillators. CRMs have long been used in the treatment of bradycardia and tachycardia, but only more recently CRMs have been specifically designed to enhance the hemodynamic performance of the heart for patients suffering from CHF. Pacemakers designed for treatment of bradycardia have incorporated a variety of physiologic sensors whereby the pacing rate of the pacemaker can be made to vary automatically with changes in physiologic demand. Thus, for example, the Hauck et al. U.S. Pat. No. 5,318,597 describes a rate adaptive pacemaker in which the pacing rate is adjusted in accordance with changes in a patient""s minute ventilation. Means are provided in the CRM for measuring variations in transthoracic impedance and for processing of the impedance signal to extract a minute ventilation signal that is used to vary the pacing rate of an implantable pacemaker between lower and upper programmed rate limit values in that there is a direct correlation between a patient""s minute ventilation and the body""s hemodynamic demand.
Other rate adaptive CRMs have incorporated some form of an activity sensor, such as an accelerometer, for developing a control signal that varies with a patient""s level of physical activity. This control signal is then used to vary the pacing rate of a rate adaptive CRM device to maintain an appropriate pacing rate for the level of exercise being exerted.
In implantable CRM devices especially designed for treating patients with CHF, one approach that has proved successful involves the automatic optimization of the AV delay of an implantable, dual-chamber pacemaker. For a general description of the prior art relating to pacemakers for treating CHF, reference is made to the Baumann U.S. Pat. No. 5,800,471, the teachings of which are incorporated herein by reference as if set forth in full.
The present invention constitutes a departure from the related art in that even though the apparatus employed incorporates circuitry for implementing impedance plethysmography in generating a signal component relating to minute ventilation and an accelerometer responsive to patient activity, the information derived from these sensors is used to monitor a CHF patient""s status so that the efficacy of a change in pacing therapy or drug therapy can be read from the implanted device periodically for review and analysis by a medical professional.
Studies have shown that patients with chronic heart failure are limited by exertional dyspnea and exercise intolerance. Such patients often exhibit elevated ventilatory response to exercise, which can be characterized by a steeper slope relating minute ventilation to carbon dioxide output during exercise. In addition to the increased ventilation, such patients have also been noted to have an abnormal breathing pattern, such that at a given minute ventilation, respiratory rate is increased while the change in tidal volume is less significant compared with normal subjects. The ventilatory response to exercise, as characterized by the regression slope relating minute ventilation to carbon dioxide output during exercise by CHF patients, has been found to be significantly higher in such patients than for normal subjects. See xe2x80x9cThe Role of Peripheral Chemoreflex in Chronic Congestive Heart Failurexe2x80x9d by T. P. Chua et al, CHF; November/December 1997; pp. 22-28.
The method of the present invention is carried out by implanting in the patient a CRM device of a type incorporating a transthoracic impedance sensor, a patient activity sensor and a processor operatively coupled to the transthoracic impedance sensor for controlling the delivery of cardiac stimulating pulses to the patient""s heart. The processor is programmed to permit it to derive a plurality of respiratory parameters from signals produced by the transthoracic impedance sensor. A telemetry link may also be provided to allow the respiratory parameters or signals relating thereto to be transmitted to an external monitor for viewing by a medical professional. Either the processor forming a part of the implanted CRM device or a processor contained within the external monitor computes predetermined ratio values involving the derived respiratory parameters as congestive heart failure therapy status indicators.
The apparatus involved may further include an activity sensor connected to the microprocessor for deriving a plurality of physiologic parameters based upon the signal output of the activity sensor. The activity sensor derived parameters may also be telemetered to the external monitor for display or for further processing and display.
Meaningful ratios that serve as CHF status indicators include: ventilatory tidal volume to respiratory rate (Vt/RR); ventilatory tidal volume to inspiratory time (Vt/Ti); ventilatory tidal volume to expiratory time (Vt/Te); minute ventilation to carbon dioxide output (MV/VCO2); respiratory rate to minute ventilation (RR/MV); and tidal volume to minute ventilation (Vt/MV). Another ratio of interest is O2 pulse, which is the amount of oxygen uptake per heartbeat (VO2/HR).
Other researchers have found a high correlation between oxygen consumption (VO2) and carbon dioxide production (VCO2) with heart rate and physical activity to the point where it is possible to estimate these respiratory parameters based on measured heart rate and physical activity. See, for example, xe2x80x9cCombined Heart Rate and Activity Improve Estimates of Oxygen Consumption in Carbon Dioxide Production Ratexe2x80x9d by Jon K. Moon and Nancy F. Butte, J. Applied Physiology, 81(4:1754-1761) 1996. The present invention takes advantage of such high correlation, providing a way in which a particular pacing or drug therapy affects the status of CHF patients in whom the present invention is utilized.
The present invention provides a method and apparatus for monitoring a patient""s status of congestive heart failure. The method is carried out by first implanting in the patient an electronic device capable of measuring transthoracic impedance and for sensing a level of physical activity. The transthoracic impedance signal is then processed such that an estimate is derived of the patient""s minute ventilation, respiratory rate and tidal volume. Likewise, the oxygen uptake (VO2) and carbon dioxide production values (VCO2) are estimated from the accelerometer measured activity. It has been found that by calculating a ratio of tidal volume to respiratory rate, a first CHF status indicator is obtained.
Also, once tidal volume information from a patient is arrived at, his or her inspiratory time can also be derived. The ratio of tidal volume to inspiratory time is found to comprise a second congestive heart failure status indicator that is meaningful. The ratio of minute ventilation to carbon dioxide production can be derived as a third congestive heart failure status indicator. Similarly, the ratio of oxygen uptake to heart rate is found to be a further CHF status indicator.
The invention also contemplates the use of the CHF status indicators (ratios) in the closed-loop control of a CRM device that for example may adjust a pacing parameter to optimize the benefit afforded by the pacing therapy.
The foregoing features, objects and advantages of the invention will become apparent to those skilled in the art from the following detailed description of a preferred embodiment, especially when considered in conjunction with the accompanying description and drawings.